1. Field of the Invention
This invention relates to the field of downlink systems and more specifically to downlink systems for communicating with a downhole tool in a wellbore.
2. Background of the Invention
Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a drill string. The well is drilled so that it penetrates the subsurface formations containing the trapped materials for recovery of the trapped materials. The bottom end of the drill string conventionally includes a bottomhole assembly that has sensors, control mechanisms, and associated circuitry. As the drill bit is advanced through the formation, drilling fluid (e.g., drilling mud) is pumped from the surface through the drill string to the drill bit. The drilling fluid exits the drill bit and returns to the surface. The drilling fluid cools and lubricates the drill bit and carries the drill cuttings back to the surface.
During drilling, communications between the surface and the bottomhole assembly may be performed using a telemetry system, which allows for communications such as power, data, and directional commands between the bottomhole assembly and surface devices. Typical telemetry systems include a downlink, which is a communication from the surface to the bottomhole assembly. For instance, based on the data collected by the sensors in the bottomhole assembly, an operator may desire to send a command to the bottomhole assembly such as to change the drilling direction. Telemetry systems also include an uplink, which is a communication from the bottomhole assembly to the surface. For instance, an uplink may be a transmission of the data collected by the sensors in the bottomhole assembly such as orientation data or a confirmation that a downlink command was correctly received.
A conventional method of telemetry is mud pulse telemetry. Mud pulse telemetry is a method of sending signals, either downlinks or uplinks, by creating pressure and/or flow rate pulses or waves in the drilling mud. The pulses may be detected by sensors at the receiving location. For instance, in a downlink operation, a change in the pressure or flow rate of the drilling mud pumped down the drill string may be detected by a sensor in the bottomhole assembly. The pattern of the pulses, such as the frequency, timing and the amplitude, may be detected by the sensors and interpreted so that the command may be understood by the bottomhole assembly. Drawbacks to mud pulse telemetry include the temporary interruption of drilling operations to allow mud pumps at the surface to be cycled on and off, which provides the pulses. Further drawbacks to mud pulse telemetry include that downlink signals may not be sent when the mud pumps are cycled off. Additional drawbacks include the length of time involved in sending a downlink because of the measured depth of deviated wells as well as inefficiencies in downlinks when gas is entrained in the drilling mud. Drawbacks also include the cost of surface equipment and maintenance for mud bypass modulators and the like. In addition, drawbacks include the time that it takes to send information to the downhole tool and the surface equipment cost. For instance, to send a downlink command, the mud flow rate or the pressure is modulated. Such modulation is sensed by the sensors in the downhole tool. Because the mud pumps are large mechanical systems, a large inertia is involved, and therefore the flow rate may not be changed quickly, which results in an inefficiently long time to send the downlink commands. Another method to modulate the flow rate going downhole is to by-pass the flow from the stand pipe back to the mud pits, which is modulated by the surface modulator or valve. Drawbacks to such method include the considerable time and increased cost of the surface equipment, installation and maintenance. It also poses safety risks on the rig floor and added costs to mitigate the safety issues.
Another conventional method of telemetry is electromagnetic telemetry, which is a method of using electromagnetic telemetry tools to send the uplinks and surface equipment to send downlinks by injecting modulated current in the earth formation. Drawbacks to the conventional electromagnetic telemetry systems include that the downlink signals reaching the downhole tool are typically weak and may not be sensed while the uplink telemetry is being transmitted. Relatively small signals of the downlink may be overcome by large uplink signals transmitted by the downhole tool.
Further drawbacks in many cases of mud pulse telemetry and electromagnetic telemetry include deployment of special sensors in the downhole tool to detect signals, which may increase the cost of the system.
Consequently, there is a need for an improved telemetry system that allows for downlinks when mud pumps are cycled off or when the uplink telemetry is in progress. Further needs include improved detection and transmission of downlinks. Additional needs include an increase in the speed of sending downlinks. Further needs include a downlink telemetry system with reduced surface and downhole equipment cost and reduced maintenance.